Electrodeposition coating is a method wherein aqueous paint is filled in a tank and paint components are electrophoretically moved and deposited onto a surface of an object to be coated present in the tank. The main application of this electrodeposition coating is anti-corrosion coating of car bodies and it is industrially important.
In general, an electrodeposition paint contains a pigment and a pigment grinding agent. In order to introduce a pigment into paint, a pigment paste obtained by grinding the pigment in an aqueous medium at a high concentration is used as an intermediate composition. To the pigment paste, a dispersing agent is formulated to give sufficient dispersion stability to the pigment paste. The dispersing agent is required to have the characteristic of increasing the dispersion stability of the pigment paste as well as performances to keep the paint stable for a prolonged period when the pigment paste is made into an electrodeposition paint by diluting it with an aqueous medium. In recent years, a cationic resin is generally used for the pigment grinding agent of electrodeposition paint.
It is known that a sulfonium group-containing resin is superior in corrosion resistance in comparison to the other cationic resins. However, when basic pigments such as titanium oxide and basic lead silicate are dispersed with the cationic resin, dispersibility of the resulting dispersion may become poor, because an electrostatic force of repulsion between the cationic resin and the basic pigments is generated.
On the other hand, in Japanese Laid-Open Patent Publication No. 60-161464, there is disclosed a cationized acrylic copolymer in which an unsaturated carboxylic acid is used as a part of monomers for producing it. Further, in Japanese Patent Application No. 4-33141, there is disclosed that excellent stability can be obtained by introducing a carboxyl group into a resin.
However, an acrylic resin is inferior in compatibility with a cationized epoxy resin which is generally used as a main binder in electrodeposition. Therefore, when an acrylic resin is used as a pigment grinding resin and the resulting pigment paste is introduced into the electrodeposition paint, corrosion resistance of the resulting coated film becomes poor.